1. Field of the Invention
This invention relates to a drilling screw to drill and fasten workpieces such as steel plates by itself without any preparation of predrilled hole in the workpieces, and more particularly to a drilling screw to apply to the workpieces which is about 1 mm to about 2 mm in thickness.
2. Description of the Prior Art
Two different types of self-tapping screws are known for fastening workpieces together without requiring predrilling holes therefor in the workpieces. One is a cone point tapping screw, and another is a drilling screw which has drill means at a tip portion. Many screws and methods of producing them are known.
The cone point tapping screw comprises a shank section which estends at the same diameter and has threaded convolutions thereon and has a head at one end, a tapered section with tapered threaded convolutions at another end of the shank and, the tapered section terminates in a sharp tip.
The workpieces are broken through by the sharp tip and the hole produced is enlarged by the tapered section, then the drilling through the workpieces is made and the workpieces are fastened by the shank section.
In cases where the workpieces are hard and thick, drilling does not occur efficiently on the tapered section. When the workpieces are drilled, the cone point tapping screw does not move forwardly, but races, and, the tapered section is superheated and damaged. Accordingly, the drilling ability of the screw falls conspiciously.
In a conventional screw, there is an improvement wherein the tip angle is reduced less than 40 degrees and has the tapered section of a circular cone shape in addition threaded convolutions are formed on the tapered section.
The tapping screw is applied to workpieces of relatively soft materials of thin steel plates which are less than 1 mm in thickness and easy to perforate by the sharp tip portion and immediately insert the threaded shank into the hole. Therefore, the required drilling time is reduced as compared with a drilling screw which drills the workpieces by scraping. Also it is able to take the correct position by the sharp tip and to provide the tapping screw at low cost, since the structure thereof is simpler than drilling screws. However, if the tip is excessively sharpened the tip section is easily broken and it is very difficult to provide the threaded convolutions on the tapered section. Therefore the productivity is decreased.
In another drilling screw, the screw has generally a shank which has the same diameter, a head at one end, and a drill section which extends from the shank and has a chisel point at the tip and cutting edges which are symmetrically formed by cutting sections and relief flutes on the shank and drill section. The drill section of the drilling screw scrapes and simultaneously perforates the workpieces the same as a drill section of a drilling screw. The drilling screw is able to be used on workpieces to obtain equivalent drilling efficiency by perforation without regard to the materials and or thickness of the workpieces, which is different from the cone point screw.
However, there is great resistance against advancing the chisel point which is provided to reinforce the cutting edge since it is the same as rotation of an obtuse angle wedge. Therefore, the drilling efficiency is decreased, and the tip of the drilling screw easily slips out of the hole as compared with the cone point screw, and productivity of the drilling screw decreases. Also the cost for manufacturing thereof increases for the reason that the complex structure requires curvature processing in order to be similar to the cutting edge of a drill. Under these circumstances, in a case where the materials of the workpieces are soft or the thickness thereof is less than 1 mm even if the materials are hard, it is better to use the low cost cone point screw which takes a shorter time for drilling than the drilling screw. On the other hand, in a case where the workpieces are hard and are more than 2 mm in thickness, it is better to use a drilling screw to obtain equivalent sharpening perforation efficiency.